A theory is scientific if it can be falsified by a clear test, not just confirmed.
Popper noticed that anyone can easily find evidence that seems to confirm their ideas, even wrong ones. What separates science from pseudoscience is that a scientific theory risks something: it clearly states what observation could destroy it. A theory that can never be proven wrong is not truly testable.
When you say a theory is good because it has many supporting examples, you fall into a trap. For instance, if someone believes a certain stone brings luck, they will always find cases when something good happened after touching it. But they will not notice the days when luck did not come. The real strength of a theory lies in the courage to state exactly what should happen for it to be wrong. This courage is what makes science different from superstition.
Why it matters His rule remains the standard used today to judge whether research truly counts as scientific.